This is the first application filed for the present invention.
Not Applicable.
The present invention relates to the field of optical transmission systems for telecommunications and, in particular, to a method and system for measuring a signal gain produced by Raman amplifiers connected to an optical link.
Long distance data transmission in optical networks requires periodic amplification of the optical signals to compensate for attenuation due to the cumulative effects of absorption and scatter in optical fibers. One approach to optical amplification utilizes Raman optical amplifiers.
Raman amplifiers play an important role in optical communication systems because they permit longer fiber spans. They provide better signal amplification while introducing less noise than traditional signal amplifiers. The Raman amplifier compensates for fiber loss by providing signal gain in every span. It uses the non-linear scattering property of optical fiber known as Stimulated Raman Scattering (SRS) to transfer energy from pump lasers to signal channels.
Raman amplifiers can be configured as either forward Raman amplifiers or backward Raman amplifiers. Backward Raman amplifiers exhibit better performance and are used in long-haul optical networks. Backward Raman amplifiers are installed at the down stream end of an optical span fiber, amplifying the signals along the fiber. These amplifiers use the span fiber as a medium for amplification, hence they are called distributed Raman amplifiers.
The Raman effect causes light traveling within a medium, such as an optical fiber, to be amplified by the presence of shorter wavelength light traveling within the same medium. Energy is transferred from the shorter wavelength light to a longer wavelength signal. The gain spectrum of a silica fiber pumped by a monochromatic Raman pump exhibits maximum gain when the wavelength of the signal to be amplified is approximately 100 nm longer than the wavelength emitted by the Raman pump. Multiple Raman pump lasers at different wavelengths can be used to spread this influence over a wider range of longer wavelengths. Backward Raman amplifiers typically use two to four pump lasers to provide amplification for 40 to 80 signal channels.
One of the problems associated with such arrangements is the difficulty in achieving a uniform gain over a range of wavelengths. The relative powers required for each Raman pump changes as the mean gain of the Raman amplifier increases, due to the complex interactions resulting from stimulated Raman scattering between the various optical wavelengths in the fiber.
Effective use of a Raman amplifier in a communications network requires gain measurement and Raman pump control to obtain a desired gain. To control a multiple pump Raman amplifier, the gain should be mapped to individual pump powers. Calculating a theoretical relationship between Raman gain and relative pump powers requires solving a non-linear system of differential equations that describe optical signal propagation and the Raman scattering phenomenon in the fiber. However, there are too many variables and equations to be simultaneously solved. There are also practical issues to be considered such as connection losses, variations in fiber core size and attenuation, etc., to accurately or easily model this behavior.
There therefore exists a need for a mechanism that permits a network administrator to determine relative pump powers for a backward Raman amplifier in order to provide uniform gain over a range of optical signal channels used for data transfer through an optical network.
An object of the present invention is to provide a method for determining pump power values for a backward Raman amplifier in order to provide uniform gain across a range of optical signal channels. The method described here is not limited to achieve a uniform Raman gain profile and can be similarly used to provide any gain profile for Raman amplification. Two terminologies are distinguished hereinafter; xe2x80x9cRaman gainxe2x80x9d means the gain as a function of channel wavelength, and xe2x80x9caverage Raman gainxe2x80x9d means the average of all gains obtained for each wavelength.
According to an aspect of the present invention, there is provided a method for determining a power setting for each of a plurality of Raman pumps of a Raman amplifier used to enhance signal propagation in a given fiber medium, the method comprising steps of: pre-computing table values for a look-up table, the table values representing, for each of a plurality of average Raman gain values, corresponding normalized power values for each Raman pump required to provide substantially uniform Raman gain across a plurality of signal wavelengths, by solving predetermined fiber propagation equations; determining a linear relationship between a total Raman pump power and an average Raman gain in the given fiber medium at the initialization of the Raman amplifier in the system; determining a required total Raman pump power to achieve a desired average Raman gain using the linear relationship; looking up a normalized power value in the look-up table corresponding to the desired average Raman gain for each Raman pump; and multiplying the normalized power value for each Raman pump by the required total Raman pump power to determine the power setting for the respective Raman pump.
In accordance with another aspect of the present invention, there is provided a Raman amplifier comprising: a plurality of Raman pumps respectively outputting a predetermined range of optical wavelengths; a controller for controlling a respective pump power for each of the Raman pumps, the controller being responsive to the input of a desired average Raman gain to compute the respective total pump power value, and using a look-up table that provides normalized pump powers for the respective Raman pumps to provide a uniform gain distribution (or any given gain distribution).
The present invention uses a combination of theoretical calculations and empirical measurements to provide a method for controlling a backward Raman amplifier in an optical communication network. The theoretical calculation is used to construct the look-up table which relates the average gain to normalized power values whereas the empirical measurement determines the relationship between the average Raman gain and the total Raman pump power.
It has been shown that there is a linear relationship between total Raman pump power and the average Raman gain. There is also a linear relationship between the signal power injected into the far end of the fiber, the signal power coming from the fiber, span fiber loss and the average Raman gain. Therefore, a change in average Raman gain equals change in signal power coming from the fiber. The latter can be measured directly by an optical spectrum analyzer (OSA) or indirectly by devices built into optical transmission equipment.
The Raman effect induces coupling between optical energy at various wavelengths. This includes pump-to-signal, signal-to-signal and pump-to-pump. The propagation of optical energy at various wavelengths in optical fibers can be described theoretically by a system of non-linear differential equations.
It has also been shown that, although solving these equations does not give an accurate measure of absolute Raman gain in a real network, it does provide an accurate representation of the effects of relative changes in individual pump powers on the shape of the Raman gain curve for a specific average Raman gain. For each average gain the look-up table can therefore be filled with the required normalized pump powers (pump power ratios) in order to have a uniform Raman gain for each wavelength for the given average Raman gain. Normalized pump powers remain constant over a region of average Raman gains and therefore high resolution in look-up table is not required.